This invention relates to tuned sound barriers for machines such as electrical transformers, which radiate sound at a few constant, discrete frequencies. The sound barrier of the instant invention is tuned to block transmission of sound at the particular discrete frequencies emitted by the machine within the sound barrier.
The noise radiated by electrical machines such as power transformers has two distinct sources. One source is the cooling fans generally employed with such machines: the cooling fans produce broadband noise, along with some blade passage tones. The second source is the transformer core, the core noise is tonal, with components at twice line frequency, i.e., 120 Hz, and its harmonics. The dominant noise source is the purely tonal noise coming from the core with tones at 120 Hz, 240 Hz, 360 Hz, etc. These low frequency tones are very strong, and dominate the total audible transformer noise, despite the reduced sensitivity of the humar ear to low frequencies. In a power transformer, the source of these tones is magnetostrictive vibration of the core, which is coupled into the transformer case via the core support structure and other elements of the transformer, causing vibration of the housing surrounding the transformer at twice line frequency and harmonics thereof, which transmits the low frequency noise into the surrounding environment.
Several prior art attempts have been made to reduce transformer noise, all associated with considerable cost penalties. One method is to derate the transformer. By reducing the flux density in the core, the magnetostrictive forces in the core are reduced. Since it is the magnetostrictive forces which cause the core to vibrate at twice line frequency (120 Hz for a 60 Hz system), and its harmonics (240 Hz, 360 Hz, etc.), derating the transformer reduces the source of vibration. However, derating the transformer means that a larger and more expensive transformer is needed for a given system electrical requirement. Another prior art technique of noise control is to quarantine the transformer in the middle of a large compound. This method exploits the reduction in sound intensity as sound propagates away from a source. The reduction in sound intensity obtained is about 6 decibels for each doubling of distance, so that a significant reduction in noise levels at the boundary of the compound requires a large area. Yet another technique of noise control is to build an external barrier or block house around the transformer. Heavy weight masonary barriers or even complete enclosures can be effective sound barriers; however, at the low frequencies at which a transformer operates, a conventional barrier must be massive to be effective. If a partial barrier (i.e., a wall) is employed, it must be very high to eliminate sound diffraction over the top, and a complete block house poses problems of access to and reduced cooling of the equipment enclosed. Another technique is to employ close-fitting shells including various forms of double wall construction and close-fitting mechanically-isolated panels. These offer some degree of noise reduction, but add complexity and considerable cost. With the exception of derating the transformer, the prior art techniques operate by reducing transmission of sound between the source in the transformer and the listener and are indifferent to the frequency of the sound emitted by the source.